1. Field of the Invention
The present invention relates to a process for the manufacture of a tubular camshaft, useful, e.g., for an internal combustion engine. In accordance with the process individual cams are attached to a prefabricated tubular shaft.
2. Description of the Prior Art
Camshafts are usually solidly cast; the cams themselves are then worked to specified dimensions through lathing and grinding. In recent years, in response to multivalve technology, light-weight, concave camshafts have been needed. These camshafts promote one-shot lubrication and reduce material costs. The first practical attempts at fabricating camshafts from individual segments have recently been made. This fabrication involves assembling individual tubular shaft segments together with prefinished cams to form the entire shaft, or provides for attaching individual cams to a single-piece corrugated tube and connecting the cams thereto by cementing, soldering or mechanical means. Many processes which provide for connecting the concave-shaped shaft and prefabricated cams and, if required, bearing elements have been previously described in the art.
The most common of these processes include
Shrink fitting the cams onto the tubular shaft,
Threading the cams onto the shaft and subsequently expanding the tube by suitable compressive means, for example, by an explosion-like high-speed deformation (DE-AS 22 32 438),
Thermal expansion with concurrent upsetting of the tube by clamping jaws attached to the ends (DE-OS 34 31 361), and
Combined shrink-fitting of the cams and elastic expansion of the tubular shaft (DE-AS 26 57 479).
Among other methods, a specific process previously disclosed (DE-OS 34 31 361) involves attaching the cams to the shaft by soldering in conjunction with tube expansion. According to this method, in order to bolster the clamping effect and to achieve an extremely rigid solder connection, the cams are provided on their interior peripheral area with a notched gear-and-tooth configuration.
Such techniques are becoming more and more significant because the automobile industry, owing to heightened emission control limitations, is introducing more engines having four or more valves per cylinder, which means that the number of cams per shaft or per engine is thereby commensurately increased. Even today, cast camshafts, finished by means of lathing and grinding, are produced more economically than camshafts with cams that have been threaded on or otherwise mounted. The new technique of the assembled camshaft, however, provides substantial advantages with respect to the practical further development of the camshaft-regulated internal combustion engine. This technique also offers benefits with respect to the choice of materials for and molding of the camshaft.
A formcast camshaft consists of a uniform material. Cams exposed to particular wear and tear frequently undergo additional surface processing and treatment, receiving a particularly abrasion-resistant protective surface coating. In contrast to this, tubular shafts bearing mounted cams can be constructed using different materials for both parts (DE-OS 23 36 241).
The aforementioned patent disclosure specifies, for example, the use of sintered, sinter-forged, cast, extruded, stamped or even lathed and milled parts for the cams attached to the tubular shaft. It proposes solidly attaching these cams to the shaft by means of cementing, welding, brazing, shrinking or expanding.
A drawback inherent in all the processes described hereinbefore is that the subsequent attachment of cams to a shaft is difficult due to both the great technical expense and time implicit in the preparatory treatment of the cams and in their exact positioning on and joining to the shaft. The process coordination involved in expansion or shrink-fitting with respect to the materials of choice used in the tubular shaft and in the cams has also not been fully resolved from a technical standpoint. This selection of materials entails substantial compromise.
Finally, expansion of the tubular shaft results, as a rule, in the flowing of materials and, consequently, in forming varyingwall thicknesses in partial sections of the tubular shaft. Allowance must be made for these irregularities when dimensioning the tubular walls. That is, in order to assure satisfactory physical properties, relatively thick-walled tubes must be used. This, however, runs counter to the need to develop the lightest possible camshafts for fuel-efficient internal combustion engines.